A Preliminary Study on the Evaluation Method of "Expansion Scale" for Particleboard

Abstract: In recent years, particleboard has encountered many quality issues related to "expansion" when used in large-format customized home furnishings. In order to standardize the quality of particleboard and promote its application, this paper introduces two current domestic and international standards for testing the dimensional stability of wood-based panels. The changes in the width and thickness of three groups of different types of particleboard are tested according to the standard methods. The influence of the test methods on the test results is analyzed, in order to provide a reference for the design of rapid test methods and indicators for the dimensional stability of particleboard products.

Keywords: particleboard; expansion gauge; dimensional stability; standard

Discussion of Standard Methods for evaluing Linear Expansion in Particleboard

Gao Li, Luo Shupin, Lyu Bin, Chang Liang

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China

Abstract: This study addresses quality issues related to the linear expansion of particleboard in interior applications. Two existing standards for testing the dimensional stability of wood-based panels were introduced, and three types of particleboard were evaluated using these methods. The influence of testing methods on the results was analyzed, providing a reference for developing dimensional stability specifications for particleboard products.

Key words: particleboard; linear expansion; dimensional stability; standard

As an important type of man-made board with high resource utilization, particleboard has been widely promoted and applied around the world. In 2023, my country's particleboard consumption reached 41.16 million cubic meters, most of which was used for interior custom home furnishings [1]. However, in recent years, the popular minimalist style design uses a large number of large-sized components, such as door panels and wall panels that extend from one door to the top. The quality problem caused by the "expansion" of particleboard reported by users has become a prominent problem affecting the image of particleboard products [2].

"Expansion," commonly known in the industry as the expansion of a board's length and width due to changes in ambient temperature and humidity, is referred to as linear expansion in international standards. Biomass materials such as wood, bamboo, and straw, the primary raw materials for particleboard, are susceptible to moisture absorption and desorption in the environment, causing expansion or contraction, which in turn leads to dimensional changes.

In the product standard GB/T 4897-2015 "Particleboard," dimensional stability, related to the "expansion" of the sheet material, is a property that is subject to negotiation based on the test method and index value required by both the supplier and the buyer. There is no specific index value. Instead, the method used is Section 4.35, "Determination of Dimensional Stability - Method 1," of GB/T 17657-2022, "Test Methods for Physical and Chemical Properties of Wood-Based and Veneered Panels." The widespread use of large-scale components has highlighted the severity of the "expansion" issue, and the home furnishing industry urgently needs appropriate methods and means to control the various losses caused by "expansion." Therefore, this article introduces the characteristics of two currently used standard methods for dimensional stability testing of wood-based panels, both domestically and internationally. Then, based on these standard methods, the dimensional stability of three particleboard samples was evaluated and analyzed, providing a technical reference for the development of rapid evaluation methods and indicators for particleboard "expansion."

1. Dimensional stability evaluation methods and indicators

1.1 Determination of dimensional stability - Method 1

GB/T 17657-2022, "Determination of Dimensional Stability - Method 1," is the primary method for testing the "expansion" of the length and width of particleboard, fiberboard, and their products. It measures the dimensional change of specimens due to changes in relative humidity at 20°C. The method involves cutting eight specimens, four each in the transverse and longitudinal directions, from the sample, each measuring (300±1) mm in length and (50±1) mm in width. The specimens are then divided into two groups (each consisting of two transverse and two longitudinal specimens). Each group undergoes either a humidification (Group 1) or drying (Group 2) treatment according to the conditions specified in Table 1. The rate of change in step 3 relative to step 2 is calculated, i.e., the rate of dimensional change relative to 65% relative humidity. The average of the length and thickness change rates for the four specimens in the same group represents the rate of change for that process. The sum of the absolute values ​​of the two groups represents the rate of change for the entire process (30% to 85% relative humidity).

Table 1 Balancing procedure for dimensional stability specimens

Factors affecting the dimensional stability of particleboard include the anisotropic properties of wood, the length, thickness and other morphology of the particles, the water resistance and strength of the adhesive, and the preparation process parameters. Method 1 evaluates the dimensional changes of particleboard with humidity changes under the state of moisture content equilibrium, reflecting the shrinkage and swelling characteristics of the particleboard itself after combining the above factors. This method is derived from the European standard EN 318 Wood-based panels-Determination of dimensioThe European standard for particleboards is based on the dimensional stability of the board. The dimensional stability of the board is determined by the size of the sample (16) and the size of the sample (8) of the national standard. ...

In the European standard EN 12871: 2013 Wood-based panels - Determination of performance characteristics for load-bearing panels for use in floors, roofs and walls, a requirement is put forward for the linear expansion rate of load-bearing panels used for floors, wall panels and roof panels not exceeding 4 mm/m. The specific method is to test the linear total rate of change from 30% to 85% relative humidity according to standard EN 318, that is, the sum of the absolute values ​​of the two groups of results (humidified (group 1) and dry (group 2) in Table 1).

American Standard ASTM D 1037 Standard Test Methods for evaluing Properties of Wood-baThe method described in Section 24 of the ASTM Fiber and Particle Panel Materials section, "Linear Expansion with Change in Moisture Content," is similar to the aforementioned "Method 1," but the main differences between the two methods include: 1) Specimen Dimensions: The ASTM standard requires a specimen width of 3 inches (approximately 76 mm) and a length of at least 12 inches (approximately 305 mm); 2) Test Method: The ASTM test standard is a temperature of 20°C and a relative humidity of 50%. Whether humidifying or drying, the ASTM standard eliminates the need for step 1 as required in Table 1, thus shortening the test cycle by approximately one-third; 3) Result Characterization: The linear expansion rate from 50% to 80% relative humidity (the linear expansion rate after equilibration at (50±2)% relative humidity and then equilibration at (80±3)% relative humidity) or the linear expansion rate from 30% to 80% relative humidity (the sum of the absolute values ​​of the change rates from 50% to 30% relative humidity and from 50% to 80% relative humidity) is used. The American particleboard product standard ANSI A208.1-2016 Particleboard proposes specific indicators for the dimensional stability of medium-density (640-800 kg/m3) and low-density (m3) particleboards, as well as flooring and flooring substrate products. It requires a relative humidity of 50% to 80%, a linear expansion rate of less than 0.4% for medium-density and low-density particleboards (used for furniture or decoration), less than 0.3% for outdoor floors, and less than 0.35% for flooring substrates, stair boards, etc.

1.2 Determination of dimensional stability - Method 2

GB/T 17657-2023's "Dimensional Stability - Method 2" is an accelerated evaluation method applicable to products such as thermosetting resin-impregnated paper high-pressure decorative laminate (HPL). It is derived from the international standard ISO 4586. Six specimens, measuring (120 ± 1) mm x (120 ± 1) mm, are first placed in a constant temperature and humidity chamber at (23 ± 2)°C and a relative humidity of (50 ± 5)% for at least 72 hours. Three of these specimens are subjected to a dry heat test, drying them in a forced air drying oven at (70 ± 2)°C for 24 hours; the remaining three specimens are subjected to a high humidity test, conditioned at a constant temperature and humidity of (40 ± 2)°C and a relative humidity of 90% to 95% for (96 ± 4) hours. The length dimensional change rates of the dry heat and high humidity groups are calculated separately, with the total dimensional change rate being the sum of the absolute values ​​of the two data sets. Method 2 significantly shortens the testing cycle by approximately 7 days.

In order to further evaluate the impact of the test method on the dimensional stability of particleboard, three particleboard products using different adhesives were selected below, and a comparative analysis of the dimensional stability test methods using Method 1 and Method 2 was performed.

2 Comparison of evaluation methods for particleboard dimensional stability

2.1 Particleboard samples

Three groups of three-layer particleboards were obtained from different manufacturers. No. 1 was E0-grade particleboard, primarily using a modified urea-formaldehyde resin adhesive, with a liquid adhesive dosage of 15.4% and a paraffin emulsion dosage of 0.6%. No. 2 was F★★★★-grade particleboard, with a small amount of isocyanate adhesive replacing part of the modified urea-formaldehyde resin adhesive in the core layer. The modified urea-formaldehyde adhesive liquid dosage was 14.3%, the isocyanate adhesive dosage was 0.3%, and the paraffin emulsion dosage was 0.8%. No. 3 was ENF-grade particleboard, primarily using an isocyanate adhesive, with an isocyanate adhesive dosage of 3.2%, a tackifier dosage of 0.9%, and a paraffin emulsion dosage of 1.1%.

2.2 Test and calculation methods

Specimens were cut from three groups of particleboards according to the aforementioned methods 1 and 2. The calculation method for the dimensional stability in the length direction of method 1 is as shown in formulas 1 and 2, and the calculation method for the dimensional stability in the thickness direction is as shown in formulas 3 and 4. However, method 1 does not require the recording of the difference in moisture content. This paper analyzes the changes in moisture content.

In formulas 1-4, l30, l65, and l85 represent the length between measurement points at 20°C and relative humidity of 30%, 65%, and 85%, respectively. Δl65,85 refers to the relative change in length from 65% to 85% relative humidity, and Δl65,30 refers to the relative change in length from 65% to 30% relative humidity. t30, t65, and t85 represent the thickness at 20°C and relative humidity of 30%, 65%, and 85%, respectively. Δt65,85 refers to the relative change in thickness from 65% to 85% relative humidity, and Δt65,30 refers to the relative change in thickness from 65% to 30% relative humidity. Similarly, Δmc65,85 and Δmc65,30 represent the difference in equilibrium moisture content from 65% to 85% (Group 1 in Table 1) and 65% to 30% (Group 2 in Table 1), respectively.

When calculating the dimensional stability when the relative humidity ranges from 30% to 85%, Δl30,85, Δt30,85 and Δmc30,85 represent the changes in length, thickness and moisture content respectively, which are the sum of the absolute values ​​of the humidification and drying results.

Method 2 only requires recording the dimensional change rate in the longitudinal direction, as shown in Formula 5. This paper also analyzes the changes in thickness and moisture content.

In Equation 5, ΔL represents the rate of change in length after dry heat or high humidity testing, and l1 and l2 represent the initial and final lengths after treatment, respectively. Calculate the thickness change using the same method, recording the difference in moisture content under different treatment conditions.

2.3 Experimental results and discussion

In order to analyze whether dimensional stability is related to the main physical and mechanical properties, Table 2 lists the main physical and mechanical properties of three groups of particleboards. Tables 3 and 4 are the results of dimensional stability tests carried out by Method 1 and Method 2, and calculated according to Formulas 1 to 5.

Table 2 Main physical and mechanical properties of particleboard

Table 3 Dimensional stability of particleboard (method 1)

Table 4 Dimensional stability of particleboard (Method 2)

Note: The length and thickness change rates are accurate to 0.05%.

The data in Table 2 show that all three groups of boards meet the performance requirements of furniture-grade (P2) particleboard in a dry state. The moisture content of all three groups exceeded 5%, with the formaldehyde-free sample No. 3 having the highest moisture content. The density of No. 1 and No. 3 boards was the same, while No. 2 had a density approximately 3% lower than the other two groups, yet exhibited the best overall performance. The thickness expansion rate, a measure of dimensional stability across the thickness of the board, was significantly greater for the E0-grade No. 1 board after absorbing water over 24 hours. In addition to the water-blocking effect of paraffin waterproofing agents, differences in the water resistance of adhesives were crucial. The addition of isocyanate adhesives improved the board's water resistance, resulting in a lower 24-hour thickness expansion rate. The water resistance of the F★★★★-grade No. 2 board, compared to the ENF-grade No. 3 board, was slightly better, significantly correlated with the amount of adhesive applied.

Table 3 uses method 1 for a total of 45 days of testing. If the difference in moisture content is not examined, the dimensional stability of the width and thickness of ENF grade particleboard No. 3 is better than that of No. 1 and No. 2. However, combined with the moisture content difference (Δmc30, 85), it can be found that the changes in width and thickness are closely related to the changes in moisture content. The use of isocyanate adhesives with good waterproof performance is conducive to reducing deformation; the moisture content of E0 particleboard changes as high as 13.0%, resulting in the most prominent change rate in width and thickness. From a temperature of 20 ° C and a relative humidity of 30% to an environment of 85%, the maximum length change value reaches 4.6 mm/m, which is similar to the board used in the dry environment in the north and sent to the humid environment in the south, resulting in a serious "expansion" problem. According to the European standard EN 12871 Wood-based panels - Performance specifications and requirements for loadbearing boards for use in floors, walls and roofs require a linear expansion rate of no more than 4 mm/m. Group 2 and 3 particleboards can still meet the dimensional stability requirements for building materials.

Table 4 shows the results of accelerated testing of particleboard after equilibration for 5 days at 23°C and 50% relative humidity, resulting in a 9-day test period. Preliminary testing revealed that Method 2 specimens required approximately 14 days to reach equilibrium. Therefore, the specimens equilibrated for 5 days had not yet reached equilibrium in terms of moisture content and stress. There are significant differences between the results of Method 2 (Table 4) and Method 1 (Table 3). In Table 4, ENF-grade particleboard No. 3 exhibited the poorest dimensional stability and the greatest drop in moisture content in the dry heat test, indicating that the moisture content of the specimens dried at 70°C for 24 hours was close to absolute dryness. Specimens with high moisture contents experience the greatest dimensional change due to the greatest change in moisture content. During the high humidity test, Board No. 3 exhibited excellent moisture resistance, with relatively minimal dimensional change. Looking at the overall changes, the moisture content of the No. 3 ENF-grade board approaches that of the No. 1 E0-grade board, but is significantly higher than that of the No. 2 F★★★★ board. The No. 3 board exhibits the greatest length change relative to the other two groups, but maintains the smallest thickness change. The dimensional stability trends for No. 1 and No. 2 boards, derived using both methods, are essentially identical. Considering only the "expansion" caused by humidification, it can be seen that, in addition to the influence of adhesive type and amount applied, initial moisture content is also a significant factor influencing dimensional stability.

Combined with the 24-h water absorption thickness expansion rate (24-h TS) in Table 2, it can be found that the dimensional stability of the board in the thickness direction has a good correlation with the 24-h TS, which is closely related to the waterproofness of the adhesive, but cannot be used to analyze the dimensional stability of the width "expansion".

The two standard methods, Method 1 and Method 2, have different ways of processing the board materials, and the results trends are not completely consistent. Method 1 reflects the dimensional changes of particleboard after the moisture content is balanced when the ambient humidity changes. It has strong repeatability and meets the requirements of standard testing. The problem is that the test cycle is long, which makes a big difference in analyzing the "expansion size" produced by actual applications. The test cycle of Method 2 is relatively short, and it can quickly evaluate the "expansion size" characteristics of the board after being treated under extreme conditions. However, there is a problem that the length of the equilibrium treatment time before the accelerated test will affect the subsequent test results. If the accelerated test is directly carried out under the initial moisture content conditions, the results are closer to the actual application situation, which may be a means suitable for rapid screening and evaluation by downstream companies. Further exploration of test methods will be carried out in the future.

3 Conclusion

In summary, "swell" is a reflection of particleboard's swelling characteristics. Controlling the moisture content of particleboard substrates in different usage environments is crucial. Reducing the temperature and humidity differences between production and usage environments is a key approach to reducing "swell." Different testing methods can address different needs, but further research is needed to determine the optimal and effective testing method and indicator settings.

References: Omitted

Citation format: Gao Li, Luo Shupin, Lü Bin, Chang Liang. Preliminary study on the evaluation method of particleboard's "expansion scale"[J]. China Wood-Based Panels, 2025, 32(1): 11-14. DOI: 10.12393/j.1673-5064.20250103

A Preliminary Study on the Evaluation Method of "Expansion Scale" for Particleboard

Gao Li, Luo Shupin, Lü Bin, Chang Liang

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China